Dehalogenation of geminal dihalocyclopropyl derivatives

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

The recent discovery that propadiene can be converted into a lithio compound, which is stable enough at low temperature to allow functionalization, has made it an important starting compound for the synthesis of derivatives. A fortunate circumstance is that allene can be obtained in 30% yield in an extremely simple 

Homologues in principle can also be prepared from RCH(Br)-C(8r)=CH2 but for the synthesis of these starting compounds three steps have to be carried out. The dehalogenation procedure has also been used in the preparation of di- and tri-fluoroal1enes.  

For some volatile aliphatic 1,2,3-trienes simple preparative methods have been b4-55 

The extremely unstable butatriene, HjC=C=C=CH2, can be obtained in more than 90% 

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